One victim of a fraudulent debt scam, who The Consumerist is calling “Steve”, figured out that it takes very little to get a scammer to believe you’ve switched your number. Simply call them back from your Google Voice number, let them bring up your information and start the process of trying to scam you, and then hang up. Let them call a few times and get through, then block their number.
Google Voice isn’t your only option. If you don’t want to block them and would prefer to let them keep calling a dead-end number, just sign up for a pretty much any VOIP service that provides you with an actual phone number (like Toktumi). Many of them are free, at least for awhile, and you can just ignore them while the scammers try relentlessly with no success.